Hydrogeophysics

Hydrogeophysics is a multi-discipline subject that incorporates the use of various geophysical methods and techniques to distinguish features within the subsurface, for the purpose of understanding hydrogeological properties and observing the environments relevant to the soil and the processes involving groundwater . Additionally the implementation of appropriate geophysically based methods can provide convenient detailed information into the complexities of subsurface processes, therefore assisting in creating hydrological models and groundwater evaluations.The field includes knowledge from disciplines such as physics, geology, statistics, geophysics, hydrology, rock physics and engineering.

Application of Geophysical Methods

Figure 1: Combination of a gravity profile (top) and geologic model (bottom) of bedrock (grey), dry sediments (yellow), saturated sediments (blue). The density of the lithology has some effect on the gravity along the profile.

Basin Delineation

Methods of basin delineation entail mapping of the boundaries of unconsolidated basins that assist in determining the volume of groundwater. Gravity methods can be applied, in which minor disturbances in the earth’s gravitational field can correlate to changes in density across the basin . A prime example can be the differences in densities when comparing dry sediments, saturated sediments and/or crystalline bedrock. Low gravitational potentials are indicative of saturated sediments at great depths. Refer to Figure 1.

Salt Water Intrusion

At coastal regions, salt water potentially intrudes landwards as freshwater and salt-water gradients become affected due to the result of high rates of pumping of groundwater reserves, as shown in Figure 2 . Geophysical methods can be applied, such that they are utilized to indicate the interface between freshwater and saltwater, in order to distinguish aquifers in coastal environments. Geophysics surveys can allow us to observe the evolution of salt-water intrusions during a specific time period with the help of time-lapse imaging. This opens a gateway for understanding seasonal variations with salt-freshwater interactions .

Figure 2: The image shows the mitigation of salt groundwater landwards as a result of the pumping of groundwater supplies .

Application of Geophysical Technology

Heat tracing

There is emerging technology in the use of high-resolution (HR) handheld thermal imaging cameras for investigating groundwater and surface water in environments that provide interesting thermal signatures. Temperature is an important water-quality parameter and the contrasts in temperature between water flows and surrounding sediments can be observed to detect groundwater movement. This can assist hydrologists and hydro-geophysicists in understanding the mechanisms involved in the hyporheic zone.

Continuous Resistivity Profiling (CRP)

CRP is a technology, utilized to image electrical properties of estuarine, riverine or lacustrine environments within the subsurface. Using this technology, the apparent resistivity of the environment is collected and inverted to produce the spread of the true resistivity of the subsurface in the targeted region . With this application, the resistivity data can assist in interpreting the structure and geology of the subsurface. Figure 3 provides a resistivity profile of the subsurface, acquired from using CRP technology.

Figure 3: The image illustrates the pattern of resistivity obtained from an inverted Continuous Resistivity Profile (CRP) from Lake Travis, Austin, Texas. Warm temperatures indicate high resistivity .

Electromagnetic Induction in Hydraulic Conductivity

Instruments that involve properties of measuring Electromagnetic Induction (EMI) can obtain noninvasive and dimensionally concentrated data for the purpose of characterization of soil and groundwater features . EMI tools of multiple frequencies can provide data that is inverted to produce estimations of electrical conductivity. These estimations comply with varying depths; hence the electrical conductivity is a function of depth. Additionally, there is an electromagnetic response to soils that is dependent on particle size of the soils, soil mineralization, water content and salinity . Studies shave shown that in regions where increasing clay content is associated with decreasing hydraulic conductivity, the apparent electrical conductivity seems to have an inversely proportional relation to hydraulic conductivity .

Borehole-to-Surface DC Resistivity Method

This method is applied in fracture mapping in regions of fractured rock aquifers (FRAs). Analyzing the data obtained from geophysical surveying and mapping can indicate the nature of the parameters involved in controlling the transport of fluid through fractured rock. Since is difficult to apply traditional hydraulic methods to estimate groundwater flow, the knowledge of subsurface water processes in these environments is not substantial and needs the development of better techniques to acquire the necessary information. This Borehole-to-surface DC resistivity technique consists of installing a current producing electrode into a borehole beneath the water table . Situating a current sink electrode as a receiver, some great distance away completes the circuit. Equipotential surfaces generated from the source electrode will be produced and eventually experience distortion upon interacting with a heterogeneous or anisotropic medium as a result of electrically conductive fractures in the studied subsurface environment. As a result it is possible to provide the preferential direction of hydraulically conductive fractures by measuring the electrical anisotropy using the source and receiver electrode pair.

External Links

http://wp.unil.ch/linde-hydrogeophysics/files/2015/11/Accepted_Treatise.pdf

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1002/2015WR017016

https://www.researchgate.net/publication/228648418_Hydrogeophysics_-_Opportunities_and_Challenges